Biological systems are widely used throughout the world to treat wastewater. Many variations are found in biological wastewater treatment systems. Perhaps the most popular biological system being used throughout the world is referred to as an activated sludge system. Activated sludge systems typically include a number of reactors and a clarifier that together perform secondary treatment on the wastewater. Anaerobic, aerobic and anoxic processes can be carried out in these reactors. Generally, the clarifier is positioned downstream from the reactors and separates clarified effluent from sludge. In an activated sludge process, at least a portion of the sludge is recycled to one or more upstream reactors and the biomass associated with the recycled sludge is operative to treat the water by removing such contaminants as nitrogen, phosphorus, BOD, etc.
Over a period of time, many conventional biological wastewater treatment systems reach a point where the wastewater treatment system is operating at or near capacity of their design flow rates. This is a particular problem where areas served by the wastewater treatment system experience heavy or abnormal rainfall. In some cases, a rainstorm can release so much water to the wastewater treatment system that the system is severely overloaded with excess influent flows.
Municipalities and other regional wastewater entities are challenged when it comes to dealing with this problem. There are a number of options, but most options are expensive and require substantial time to implement. One approach is to build a new wastewater facility to handle the excess flows. Another option is to add on to or modify the existing wastewater treatment facility. Again, all of these options require substantial capital expenditures.
In many cases, the excess capacity that a conventional biological wastewater treatment system is required to accommodate comes from stormwater. Generally, the nutrients and contaminants that have to be removed from stormwater are not as extensive as is wastewater from other sources, such as homes and businesses. In fact, in dealing with stormwater runoff, one of the most limiting factors in a conventional biological wastewater treatment system is clarification. Recognizing that clarification is a substantial limiting factor in a conventional biological wastewater treatment system, it has been proposed to combine a ballasted flocculation system with a conventional biological or activated sludge system where the ballasted flocculation system provides additional clarification capacity. See the disclosures found in U.S. patent application Ser. No. 11/086,849 entitled “Method and System for Utilizing Activated Sludge in a Ballasted Flocculation Process to Remove BOD and Suspended Solids”, and U.S. patent application Ser. No. 11/331,540 entitled “Water or Wastewater System and Method for Removing BOD and Suspended Solids Through an Activated Sludge Process and a Ballasted Flocculation Process”, both of which are owned by the Assignee of the present invention, I. Kruger Inc. of Cary, N.C. USA. While ballasted flocculation systems can perform in conjunction with conventional activated sludge systems, combining the two approaches presents many challenges. After all, ballasted flocculation systems have, for the most part, been used in treating drinking water—not wastewater. In systems that combine biological treatment and ballasted flocculation treatment, treating the wastewater passing through ballasted flocculation portions of the total system presents different challenges and concerns then those typically presented when the ballasted flocculation system treats drinking water.